The bigger the horn, the better the mate

Male rhinoceros beetles (Trypoxylus dichotomus) wield an enormous pitchfork-like horn on their heads, which they use in battles with rival males over access to females.
[Image courtesy of Will Freihofer and Douglas Emlen]

In the past, researchers have generally assumed that super-sized body parts—like beetle horns, deer antlers, and extra-long bird tails—were symbols of a male's fitness. Now, a new report has confirmed that these "exaggerated" traits are, in fact, honest signals of male quality. It also demonstrates how the growth of these super-sized body parts may be affected by nutrition and insulin signaling in the body.

Douglas Emlen from the University of Montana, along with colleagues from Washington State University and Michigan State University, studied male rhinoceros beetles that have large forked horns jutting out from their heads. Some of the beetles' horns are just tiny bumps, and others grow to be two-thirds the length of a beetle's body.

The researchers performed gene-silencing experiments and discovered that the beetles' horns were more sensitive than other body parts, such as the beetles' wings, to changes in insulin and the insulin-like growth factor pathways. (Insulin and insulin-like growth factors are known to control tissue growth and body size in many different animals.)

So, in light of these findings, Emlen and his colleagues suggest that an increased sensitivity to these two pathways may be responsible for the exaggerated growth of rhinoceros beetle horns. And since the growth of rhinoceros beetle horns now seems to be linked to nutrition (insulin)—and because the size of beetle horns vary significantly from beetle to beetle—the researchers suggest that these super-sized body parts really do reflect the quality of an individual.